There is a ferry that goes almost constantly from Dakar to Goree and retour. It starts at 6:15 and stops at midnight, so you won't have problems in getting there and back.
If you wanna visit the museums... they open at 10:30, so don't take any ferry before 9:30 unless you want to wait there at the island till they open...
5000 CFA (7,5 euros) for foreigners, the return ticket.
In Dakar, go to the harbour and once inside, turn right till you find the signs for the ferry.
The ferry to Goree takes about 30 min or so either way. There are basically two identical boats that make the trip from Dakar. Don't know if you can see the sched in the pic but the boats fun every 30 min to an hour during the day.
The only way to reach Goree Island is by ferry. Ferries leave the port of Dakar in regular intervals and take about 15 minutes to cross the bay of Dakar. Fees are around CAF 5000.
Just walk around and you will find everything you need. The island is not very big, but big enough for a couple hours of walking and stopping to visit places and take pics.
The ferry goes about every 1 to 2 hours, takes about 30min and is very reliable and safe.
It costs about 5000 CFA (€7,5 / $10) for an adult and 2000 CFA for a child.